DNA tests show that a SARS patient in China may have picked up the virus from a civet cat in December 2003.Image courtesy Rebecca Postanowicz/Lioncrusher's Domain.

Chinese scientists are reporting that a person hospitalized with SARS just last month may have contracted the virus from an animal rather than a person. DNA from the patient’s virus matched that of SARS-like viruses isolated from palm civet cats more closely than it did viral DNA isolated from any humans with SARS.

The finding strengthens the argument that the SARS epidemic originated in animals, the researchers report today online in the journal Science. It is part of a larger study by the Chinese SARS Molecular Epidemiology Consortium that tracked genetic changes in the virus during three stages of the epidemic.

Apparently, the virus successfully adapted to humans and became more infectious over time, the researchers conclude. The study underscores the importance of detecting infections early on as a way to contain the disease.

“When the [SARS] virus invaded the human population, it went through phases of adaptation, and it gradually found a comfortable niche in the human population,” says Chung-I Wu of the University of Chicago in Illinois, who was a member of the research team.

As the virus evolved, it became more infectious. Early on, only one percent of people that came in contact with infected people became infected themselves; later, the rate of infection was nearly 70 percent.

In the study, the researchers compared the genomes of about 60 SARS viruses isolated during the early, middle, and late stages of the epidemic. Several SARS-like viruses from Himalayan civet cats were also part of the analysis.

The SARS virus went through three distinct phases during its evolution in humans.

In the early phase (November 16, 2002-January 31, 2003), SARS viruses had identical genetic fingerprints to those in animals such as civet cats, providing evidence that the virus had jumped from animals to people. Seven of the 11 early patients had contact with wild animals.

The middle phase of the virus’s evolution (January 31-February 21, 2003) was characterized by more person-to-person infections. Viruses from the middle phase all had a common genetic fingerprint.

By the late phase (after February 2003), the SARS virus had fully adapted to living in humans.

The person hospitalized last month with SARS was a television producer in the city of Guangzhou. How he contracted the virus is not clear, but his case indicates that there’s still a danger of SARS-like viruses moving from animals to people.

He, J. F- et al. Molecular Evolution of the SARS Coronavirus During the Course of the SARS Epidemic in China. Sciencexpress (Published online January 29, 2004).